That's the original grip. For cheaper bodies I'd buy a third party one but on a 5D3 I wanted the original grip.

By the way: You can take off the shoulder pad of that strap and then (depending on how thin you are) the camera can be carried like a belt. Once I removed the grip, used my small 28-105mm lens instead of the 24-105mm and then I had a quite small set. I really like that strap!